Our basic need is to establish an accurate and practicable means of predicting transplant success. We are currently using the mixed leukocyte culture test (MLC) for this purpose and are very pleased with the correlation we have obtained between our test results and our graft success. Our future efforts will be directed toward an earlier reading of the MLC and toward the use of donor skin cells as a non-responding but antigenic population (MSLR). We are also studying the effects of systemic immunization, immunosuppression, and the kinetics of rejection using the MLC as a monitor.